1. Field of the Invention
The present invention is generally related to coating the internal and external surfaces of porous ceramic materials with a protective or catalytic metal oxide or ceramic layer and, more particularly, to using metal organic chemical vapor deposition (MOCVD) to apply metal oxides or other ceramics on the internal and external surfaces of a porous ceramic material without plugging the pores.
2. Description of the Prior Art
Many ceramic materials have desirable properties such as mechanical strength and heat resistance which make them ideal for use under extreme environmental conditions. Nevertheless, the surfaces of many ceramic materials must be modified to achieve corrosion and/or oxidation resistance before they can be used in hostile environments. Thin metal oxide coatings can be applied to the surfaces of ceramic materials to achieve the desired protection. Currently, there are many commonly used thin film deposition techniques for depositing metal oxide layers, including radio frequency (RF) sputtering, thermal or electron beam evaporation, and chemical vapor deposition (CVD). Variations on CVD include plasma enhanced CVD (PECVD), low pressure CVD (LPCVD), and MOCVD.
It would be desirable to have a method for coating porous ceramic materials with a metal oxide layer without clogging the pores. Catalysts with huge surface to volume ratios could be produced with such a technique. In addition, filter elements, such as high temperature ceramic candle filters, could be produced. In a candle filter, the protective metal oxide layer lining the pores could protect the filter from being degraded by the corrosive and oxidative environment in which the filter is used (coal furnace, etc ).
U.S. Pat. No. 4,533,605 to Hoffman discloses RF sputtering metal oxides such as Al.sub.2 O.sub.3, SiO.sub.2, Ta.sub.2 O.sub.5, TiO.sub.2, MgO, etc., on an article surface to impart scratch and corrosion resistance. However, sputtering is a nonconformal, physical vapor deposition technique which would be completely unsuitable for coating the internal and external surfaces of a porous material. Sputtering would tend to coat only the exposed external surfaces of a porous material with a metal oxide layer. In the case of catalyst manufacture, sputtering would not coat the internal surfaces of the porous material and there would be no advantages realized in terms of surface to volume ratio. In the case of filter manufacture, sputtering would likely cause the formation of a layer over the pores at the material surface which would block the free flow of gases through the filter. Furthermore, sputtering techniques typically have low deposition rates and require high vacuum processes, both of which reduce product throughput.
U.S. Pat. No. 5,075,160 to Stinton et al. discloses that a high temperature filter for removing particulate materials from gases, such as those produced in a coal furnace, is fabricated by coating the fibers of a felt with a thin layer of protective material. Specifically, a silicon carbide (SiC) felt such as "Nicalon", available from the Nippon Carbon Company, has its fibers coated with an additional SiC layer or an aluminum oxide (Al.sub.2 O.sub.3) layer, where the coated layer is deposited by CVD. The coated layer both protects the felt from chemicals in the fluid being filtered and serves to hold the fibers together under mechanically stressful conditions. The layers are formed by blowing reactant gases through the felt. In the case of a SiC layer, methyltrichlorosilane (CH.sub.2 SiCl.sub.3) and hydrogen (H.sub.2) is blown through the felt. In the case of an Al.sub.2 O.sub.3 layer, aluminum trichloride (AlCl.sub.3), H.sub.2, and carbon dioxide (CO.sub.2) is blown through the felt. After blowing the reactant gases through the felt, the felt is heated to 1200.degree. C. to create SiC and Al.sub.2 O.sub.3, respectively, through a reaction on the felt fiber surfaces.